1. Field of the Invention
This invention relates to marine seismic systems and more particularly relates to the deployment and retrieval of autonomous seismic nodes over the back deck of a marine vessel using an overboard wheel.
2. Description of the Related Art
Marine seismic data acquisition and processing generates a profile (image) of a geophysical structure under the seafloor. Reflection seismology is a method of geophysical exploration to determine the properties of the Earth's subsurface, which is especially helpful in determining an accurate location of oil and gas reservoirs or any targeted features. Marine reflection seismology is based on using a controlled source of energy (typically acoustic energy) that sends the energy through seawater and subsurface geologic formations. The transmitted acoustic energy propagates downwardly through the subsurface as acoustic waves, also referred to as seismic waves or signals. By measuring the time it takes for the reflections or refractions to come back to seismic receivers (also known as seismic data recorders or nodes), it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits or other geological structures of interest.
In general, either ocean bottom cables (OBC) or ocean bottom nodes (OBN) are placed on the seabed. For OBC systems, a cable is placed on the seabed by a surface vessel and may include a large number of seismic sensors, typically connected every 25 or 50 meters into the cable. The cable provides support to the sensors, and acts as a transmission medium for power to the sensors and data received from the sensors. One such commercial system is offered by Sercel under the name SeaRay®. Regarding OBN systems, and as compared to seismic streamers and OBC systems, OBN systems have nodes that are discrete, autonomous units (no direct connection to other nodes or to the marine vessel) where data is stored and recorded during a seismic survey. One such OBN system is offered by the Applicant under the name Trilobit®. For OBN systems, seismic data recorders are placed directly on the ocean bottom by a variety of mechanisms, including by the use of one or more of Autonomous Underwater Vehicles (AUVs), Remotely Operated Vehicles (ROVs), by dropping or diving from a surface or subsurface vessel, or by attaching autonomous nodes to a cable that is deployed behind a marine vessel.
Autonomous ocean bottom nodes are independent seismometers, and in a typical application they are self-contained units comprising a housing, frame, skeleton, or shell that includes various internal components such as geophone and hydrophone sensors, a data recording unit, a reference clock for time synchronization, and a power source. The power sources are typically battery-powered, and in some instances the batteries are rechargeable. In operation, the nodes remain on the seafloor for an extended period of time. Once the data recorders are retrieved, the data is downloaded and batteries may be replaced or recharged in preparation of the next deployment
One known node storage, deployment, and retrieval system is disclosed in U.S. Pat. No. 7,883,292 to Thompson, et al. (“Thompson '292”), and is incorporated herein by reference. Thompson et al. discloses a method and apparatus for storing, deploying and retrieving a plurality of seismic devices, and discloses attaching the node to the deployment line by using a rope, tether, chain, or other cable such as a lanyard that is tied or otherwise fastened to each node and to a node attachment point on the deployment line. U.S. Pat. No. 7,990,803 to Ray et al. (“Ray”) discloses a method for attaching an ocean bottom node to a deployment cable and deploying that node into the water. U.S. Pat. No. 6,024,344 to Buckley, et al. (“Buckley”) also involves attaching seismic nodes to the deployment line. Buckley teaches that each node may be connected to a wire that is then connected to the deployment line though a separate connector. This connecting wire approach is cumbersome because the wires can get tangled or knotted, and the seismic nodes and related wiring can become snagged or tangled with structures or debris in the water or on the sea floor or on the marine vessel. U.S. Pat. No. 8,427,900 to Fleure, et al. (“Fleure”) and U.S. Pat. No. 8,675,446 to Gateman, et al. (“Gateman”) each disclose a deployment line with integral node casings or housings for receiving seismic nodes or data recorders. One problem with integration of the casings with the deployment line is that the deployment line becomes difficult to manage and store. The integrated casings make the line difficult to wind onto spools or otherwise store manageably. In these embodiments, the node casings remain attached directly in-line with the cable, and therefore, this is a difficult and complex operation to separate the electronics sensor package from the node casings. The use of pre-mounted node casings on the deployment line or pre-cut connecting ropes/wires between the node and the deployment line do not allow for a flexible change in adjacent node spacing/distance; any change of node spacing requires a significant amount of cost and time.
The referenced shortcomings are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques in seafloor deployment systems; however, those mentioned here are sufficient to demonstrate that the methodologies appearing in the art have not been satisfactory and that a significant need exists for the systems, apparatuses, and techniques described and claimed in this disclosure.
The existing techniques for attaching an autonomous node to a cable and deploying that cable overboard a marine vessel suffer from many disadvantages. As an example, attaching a node to a rope that is coupled to the deployment line often gets tangled during deployment and/or retrieval to the seabed. The spiraling of the tether cable can also cause problems during the retrieval when separating the node from the cable. Existing overboard chutes, ramps, and wheels also suffer from many disadvantages. First, existing overboard units have a hard time tracking movement of the cable and the cable often slips off of the overboard unit during cable deployment and retrieval. Second, while some overboard wheels may have a wheel coupled to a counterweight for passive movement, they are not active and/or powered to move, thus lacking the necessary responsiveness to changes in the movement of a deployed cable. Conventional overboard units are not able to actively change their position (such as by rotating or pivoting) when the cable moves relative to the overboard unit. This is particularly problematic in deep water applications, bad weather/sea conditions, changes in vessel direction, speed, or angle, as well movements caused by a vessel roll or changes in crab-angle. Still further, conventional overboard units are not able to deploy and retrieve nodes without causing stress and/or damage (or the potential of damage) to any attached nodes, particularly to nodes with node locks that directly attach a node to a cable. A marine vessel should be configured to efficiently deploy and recover nodes before and after their use in the water. A novel node deployment system is needed that is autonomous, limits the need for operator involvement, handling, and attaching/detaching of the nodes, and is very fast and efficient.